Conventionally, a solid fuel crushed by a crusher is supplied as a fuel by conveyance air to a boiler using a solid fuel as a fuel. The boiler includes a furnace for generating heat by burning the supplied fuel by a burner or the like and heat exchange tube banks arranged from an upper part of the furnace toward a downstream side and adapted to carry out heat exchange by causing a combustion gas to flow inside. The combustion gas discharged from the boiler is exhausted from a chimney. Here, the heat exchange tube banks are composed of an upper heat transfer unit including a secondary heater, a tertiary heater, a final heater and a secondary reheater arranged side by side at predetermined intervals in an upper part of the furnace and a rear heat transfer unit includes a primary heater, a primary reheater and a coal economizer arranged in a rear part of the furnace.
Since ash is produced from burned coal in such a boiler, the ash flows due to the combustion gas in the boiler, and causes slagging and fouling in which the ash adheres to and deposits on wall surfaces of the furnace and the heat exchange tube banks arranged from the upper part of the furnace toward the downstream side while being exhausted. If such slagging and fouling occur, heat transfer surfaces of the heat exchange tubes are covered to drastically reduce heat absorption efficiency. Further, if a huge clinker is produced on the wall surface or the like due to slagging and fouling, and the clinker falls down, problems such that a furnace pressure drastically changes, the heat exchange tube at the furnace bottom is damaged, and the furnace bottom is closed is arised. Further, since the elements of the upper heat transfer unit provided in the upper part of the furnace are arranged at narrow intervals, the furnace pressure may largely change if ash deposits. Further, the ash adhering between the heat exchange tubes grows to close a gas flow path, with the result that the combustion gas may not be able to pass, thereby causing an operational failure. Further, since the temperature of the vicinity of the wall surfaces of the furnace is high due to radiation heat of combustion flames near the burner, the ash is likely to adhere to and melt on the heat exchange tube banks having relatively low temperatures, which causes a problem that a huge clinker is likely to grow.
Accordingly, to stably operate the boiler, it is necessary to avoid the occurrence of problems caused by adhesion of ash by predicting a possibility of adhesion of ash caused by burning the solid fuel in advance. Thus, it is being attempted to indicate the possibility of adhesion of ash as an index.
For example, in non-patent literature 1, a method is used which predicts a possibility of adhesion of ash in advance based on an ash-related index based on an ash composition expressing ash containing elements in the form of oxides and an evaluation criterion. However, the index and the evaluation criterion shown in non-patent literature 1 are intended for bituminous coal, which is good quality coal with fewer problems such as adhesion of ash. As just described, since non-patent literature 1 is not intended for poor quality coals (e.g. subbituminous coal, lignite, high-silica coal, high-calcium coal and other coal types), the demand for which increases recently, there is a problem that the relationship of index and adhesion of ash disclosed in non-patent literature 1 does not necessarily tend to match the present state.
Accordingly, a technology which is intended for poor quality coals and by which coal to be used is incinerated in advance, the obtained coal ash is sintered, a conglutination degree of the sintered ash is measured and adhesion of ash is predicated and evaluated is being developed as disclosed in patent literature 1. However, sinterability and fusibility of ash is largely affected not only by temperature, but also by an ambient gas composition. In the case of a reducing atmosphere with a high concentration of a reducing gas such as CO or H2, the softening point and melting point of the ash decrease and it becomes easier to sinter. Further, in the case of an oxidizing atmosphere, the softening point and melting point of the ash increase and it becomes more difficult to sinter. Thus, the technology of patent literature 1 that does not take into consideration the ambient gas composition has a problem of having difficulty in accurately predicting adhesion of ash in a boiler.